Halometallate ligand-capped semiconductor nanocrystals

ABSTRACT

Halometallate-capped semiconductor nanocrystals and methods for making the halometallate-capped semiconductor nanocrystals are provided. Also provided are methods of using solutions of the halometallate-capped semiconductor nanocrystals as precursors for semiconductor film formation. When solutions of the halometallate ligand-capped semiconductor nanocrystals are annealed, the halometallate ligands can act as grain growth promoters during the sintering of the semiconductor nanocrystals.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a National Stage Entry of InternationalApplication No. PCT/US2016/060026 that was filed Nov. 2, 2016, theentire contents of which are hereby incorporated by reference, whichclaims priority to U.S. Provisional Patent Application No. 62/249,540that was filed Nov. 2, 2015, the entire contents of which are herebyincorporated by reference.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under DE-EE0005312awarded by Department of Energy. The government has certain rights inthe invention.

BACKGROUND

Over the past forty years, efforts to improve photovoltaics have takencrystalline silicon nearly to its theoretical limit. Second-generationsolar cells using direct gap semiconductors, such as CdTe, havedeveloped to the stage of rivaling silicon. Solution-processing offersan efficient and economically viable route to thin film CdTe-based solarcells. In particular, sintered CdTe thin films deposited from solubleCdTe nanocrystals (NCs) have proven effective as the absorber layer forCdTe solar cells.

Solution-processed solar cells made using roll-to-roll friendlytechniques have garnered increasing interest as a low-cost alternativeto single crystal silicon or chemical vapor deposited gallium arsenidethin films. A wide variety of materials have been solution processedinto photovoltaics, including CdTe. Many of the top solar cell powerconversion efficiencies (PCE) have been achieved using spin-coating toproduce a uniform semiconductor layer. However, spin-coating has manydisadvantages: significant waste of material, mediocre scalability, lowthroughput, and limited substrate geometries. All of these factors limitthe cost reduction, speed of implementation, and flexibility desiredfrom the transformation of new materials strategies into effectivetechnologies.

SUMMARY

Halometallate-capped semiconductor nanocrystals and methods for makingthe halometallate-capped semiconductor nanocrystals are provided. Alsoprovided are methods of using solutions of the halometallate-cappedsemiconductor nanocrystals as precursors for the formation ofsemiconductor films and devices incorporating the films as activelayers.

One embodiment of a method of forming a semiconductor film comprises:(a) forming a solution of halometallate-capped semiconductornanocrystals, the halometallate-capped semiconductor nanocrystalscomprising: (i) semiconductor nanocrystals, each semiconductingnanocrystal having a surface; and (ii) halometallate ligands bound tothe surfaces of the semiconductor nanocrystals; (b) coating the solutioncomprising the halometallate-capped semiconductor nanocrystals on asubstrate surface; and (c) annealing the coating, wherein thenanocrystals are sintered to form the semiconductor film.

One embodiment of a method of forming an optoelectronic devicecomprises: (a) forming a solution of halometallate-capped semiconductornanocrystals, the halometallate-capped semiconductor nanocrystalscomprising: (i) semiconductor nanocrystals, each semiconductingnanocrystal having a surface; and (ii) halometallate ligands bound tothe surfaces of the semiconductor nanocrystals; (b) coating the solutioncomprising the halometallate-capped semiconductor nanocrystals on asurface of a first electrode; (c) annealing the coating, wherein thesemiconductor nanocrystals are sintered to form a photoactive,light-absorbing semiconductor film; (d) forming a layer of chargetransporting material over the photoactive, light-absorbingsemiconductor film; and (e) forming a second electrode on the layer ofcharge transporting material.

One embodiment of a method of forming a field effect transistorcomprises: (a) forming a solution of halometallate-capped semiconductornanocrystals, the halometallate-capped semiconductor nanocrystalscomprising: (i) semiconductor nanocrystals, each semiconductingnanocrystal having a surface; and (ii) halometallate ligands bound tothe surfaces of the semiconductor nanocrystals; (b) coating the solutioncomprising the halometallate-capped semiconductor nanocrystals on asurface of a gate dielectric layer; (c) annealing the coating, whereinthe semiconductor nanocrystals are sintered to form a semiconductor filmthat provides a conducting channel layer for the field effecttransistor; (d) forming a source electrode on the semiconductor film;(e) forming a drain electrode on the semiconductor film; and (f) forminga gate electrode on the gate dielectric layer.

Some embodiments of the halometallate-capped Group II-VI nanocrystalscomprise: Group II-VI nanocrystals, each nanocrystals having a surface;and halometallate ligands bound to the surfaces of the Group II-VInanocrystals, wherein the halometallate ligands are anions having one ofthe formulas MX₃ ⁻, MX₄ ⁻, and MX₄ ²⁻, where M is an element selectedfrom group 12 of the periodic table and X is a halide atom.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawing.

FIG. 1A is a schematic illustration of a CdCl₃ ⁻ ligand exchange. FIG.1B is a schematic illustration of a method for making a photonic orelectronic device comprising an active layer made from CdCl₃ ⁻ligand-capped semiconductor nanocrystals.

FIG. 2A is a transmission electron microscope (TEM) image of CdTenanocrystals capped with tetradecylphosphonic acid. FIG. 2B is a TEMimage of CdTe nanocrystals capped with CdCl₃ ⁻ (NH₄CdCl₃) ligands. FIG.2C shows ultraviolet (UV)-visible spectra of CdTe nanocrystals ofvarious sizes capped with tetradecylphosphonic acid (dotted lines) andCdCl₃ ligands (solid lines). FIG. 2D shows Fourier transform infrared(FTIR) spectra of CdTe nanocrystals capped with native oleate ligandsand with CdCl₃ ⁻ ligands.

FIG. 3A shows a schematic exploded view (middle panel) andcross-sectional SEM images of a complete CdTe solar cell device madefrom a CdCl₃ ⁻-capped CdTe nanocrystal ink with ˜350 nm (left panel) and600 nm thick (right panel) photoactive CdTe layers. FIG. 3B shows acurrent density-voltage (JV) curve under AM 1.5G illumination for a CdTesolar cell with a ˜350 nm thick CdTe active layer. FIG. 3C shows thelight and dark JV curves under AM 1.5G illumination for a CdTe solarcell with a ˜350 nm thick CdTe active layer. FIG. 3D shows the lightexternal quantum efficiency (EQE) and internal quantum efficiency (IQE)spectra for a CdTe solar cell with a ˜350 nm thick CdTe active layer.FIG. 3E shows a JV curve under AM 1.5G illumination for a CdTe solarcell with a ˜600 nm thick CdTe active layer. FIG. 3F shows the light anddark JV curves under AM 1.5G illumination for a CdTe solar cell with a˜600 nm thick CdTe active layer. FIG. 3G shows the EQE and IQE spectrafor a CdTe solar cell with a ˜600 nm thick CdTe active layer.

FIG. 4 shows the JV curves for CdTe solar cells made with the “additive”approach and with the CdCl₃ ⁻-capped CdTe NC ink (layer thickness<400nm) approach, measured under AM 1.5G illumination.

FIG. 5 shows the JV curves under AM 1.5G illumination for spin-coated,spray-coated, and doctor-bladed CdTe absorber layers.

FIG. 6A shows the PXRD pattern for a CdTe film formed from HgCl₃⁻-capped CdTe nanocrystals. FIG. 6B shows the PXRD pattern for a CdTefilm formed from oleic acid-capped CdTe nanocrystals.

DETAILED DESCRIPTION

Halometallate-capped semiconductor nanocrystals and methods for makingthe halometallate-capped semiconductor nanocrystals are provided. Alsoprovided are methods of using solutions of the halometallate-cappedsemiconductor nanocrystals (referred to herein as halometallate-cappedNC inks) as precursors for the formation of semiconductor films anddevices incorporating the films as active layers.

The halometallate-capped semiconductor nanocrystals are semiconductornanocrystals having halometallate ligands bound to their surfaces. Thehalometallate ligands help to passivate the surfaces of the nanocrystalsand facilitate electronic communication between the nanocrystals. Inaddition, in some embodiments of the halometallate ligand-cappedsemiconductor nanocrystals, the halometallate ligands act as graingrowth promoters during the annealing and sintering of the nanocrystals.In these embodiments, the halometallate ligand-capped semiconductornanocrystals combine a grain growth promoter and a semiconductormaterial into a single entity, which enables facile device fabricationand eliminates the need for a conventional halometallate bath treatment.This is advantageous because such treatments, which have been used toimprove the efficiencies of solution-processed Group II-VI-basedphotovoltaics, are tedious, inefficient, and difficult to incorporateinto coating methods, such as spray-coating and doctor-blade coating,that provide continuous stream processing and facilitate roll-to-rollfabrication techniques. As a result, the halometallate-capped NC inkscan be used to form active layers in a variety of photonic andelectronic devices, including solar cells and field effect transistors(FETs).

As shown schematically in FIG. 1A, the halometallate ligand-cappedsemiconductor nanocrystals (shown on the right side of FIG. 1A) can bemade by replacing some, all, or substantially all of the surface cappingorganic ligands on organic ligand-capped semiconductor nanocrystals(shown on the left side of FIG. 1A) with halometallate anions. Theorganic surface capping ligands may be present by virtue of the solutionphase, colloidal synthesis of the semiconductor nanocrystals. Theseorganic ligands can include hydrocarbon chains of various lengths andhave one end bound to the surface of the nanocrystal. Examples oforganic surface capping ligands that initially may be bound to thenanocrystals include organic amines, carboxylates, and phosphonates.Specific examples of these include tetradecylphosphonic acid andoleates.

Examples of semiconductor nanocrystals that can be surface capped withthe halometallate anions include Group II-VI nanocrystals, such as CdTenanocrystals, HgSe nanocrystals, HgTe nanocrystals, HgS nanocrystals,Hg_(x)Cd_(1-x)Te nanocrystals, Hg_(x)Cd_(1-x)S nanocrystals,Hg_(x)Cd_(1-x)Se nanocrystals, Cd_(x)Zn_(1-x)Te nanocrystals,Cd_(x)Zn_(1-x)Se nanocrystals, Cd_(x)Zn_(1-x)S nanocrystals, CdSenanocrystals, CdS nanocrystals, ZnS nanocrystals, ZnSe nanocrystals,ZnTe nanocrystals, or mixtures of two or more thereof, where 0≤x≤1,including where 0<x<1. Other examples of semiconductor nanocrystals thatcan be surface capped with the halometallate anions include Group IV-VInanocrystals, such as PbS, PbSe, and PbTe. Core-shell nanocrystalshaving a Group II-VI semiconductor shell or a Group IV-VI semiconductorshell can also be used. The Group II-VI and Group IV-VI shell materialscan be, for example, selected from those listed above. Suitablecore-shell nanocrystals include those having a Group III-V semiconductorcore with a Group II-VI semiconductor shell. For exampleInAs(core)-II-VI semiconductor (shell) nanocrystals can be used.

Examples of halometallate anions that can be used as surface cappingligands include halometallates, such as chlorometallates, of metals fromgroup 12 and/or group 13 of the periodic table of the elements. Thechlorometallates may be represented by one of the formulas MX₃ ⁻, MX₄ ⁻,and MX₄ ²⁻, where M is a metal from group 12 or group 13 and X is ahalide, such as Cl or I. Specific examples of these include CdCl₃anions, CdCl₄ ²⁻ anions, CdI₃ ⁻ anions, CdBr₃ ⁻ anions, CdBr₄ ²⁻ anions,InCl₄ ⁻ anions, HgCl₃ ⁻ anions, ZnCl₃ ⁻ anions, ZnCl₄ ²⁻ anions, andZnBr₄ ²⁻ anions. The halometallate anions are desirably selected so thatthey act as grain growth promoters during the sintering of thesemiconductor nanocrystals into a thin film. The chloride, or otherhalogen, in the grain growth promoters serves as a flux during theannealing the films and result in the growth of larger grains relativeto films that are annealed in the absence of the halometallate ligands.Without intending to be bound by any particular theory of the invention,the inventors believe that the halide that remains in the film is pushedto the grain boundaries as the grains grow. By including halometallateligands that act as grain growth promoters, sintered semiconductor filmshaving mean grain sizes of 100 nm or greater can be formed. Thisincludes semiconductor films having mean grain sizes of 200 nm orgreater and further includes semiconductor films having mean grain sizesof 300 nm or greater. By way of illustration, chlorocadmate anions(CdCl₃ ⁻) can be used as grain growth promoting ligands for Group II-VInanocrystals, such as CdTe nanocrystals, as illustrated in Examples 1-3.However, the halometallate anions need not be grain growth promoters.Although, it may be advantageous to select halometallates that do notinhibit the growth of the semiconductor grains during sintering.

The exchange of the organic capping ligands with halometallate ligandscan be carried out by forming a solution containing the organicligand-capped semiconductor nanocrystals and the halometallate anionsand maintaining the solution under conditions that allow thehalometallate anions to undergo ligand exchange with the organic cappingligands. The total replacement of all organic capping ligands is notnecessary. However, a majority of the organic capping ligands aredesirably replaced. For example, from 99% to 100%, including from 99.5%to 100% of the organic capping ligands may be replaced. This can beaccomplished by forming two immiscible solutions, the first solutioncontaining the organic ligand-capped semiconductor nanocrystals and thesecond solution containing the halometallate anions. The second solutionwill generally also include counter-cations associated with thehalometallate anions. Examples of counter-cations include ammoniumcations, alkylammonium cations, and arylammonium cations, such as NH₄ ⁺,CH₃NH₃ ⁺, and C₅H₆N⁺. The two immiscible solutions can then be combinedwith optional heating, and then stirred for a time sufficient to allowthe nanocrystals to phase transfer to the second solution where ligandexchange takes place. The ligand-exchanged nanocrystals can then beseparated from the solution and collected by, for example, precipitatingthem from the solution, followed by centrifugation and/or filtration.Methods of exchanging organic capping ligands with halometallate ligandsare described in greater detail in the Examples.

As illustrated in the flow chart of FIG. 1B, solutions of thehalometallate-capped semiconductor nanocrystals (i.e.,“halometallate-capped NC inks”) 102 can be used to form films of thesemiconductor materials for use as active layers in a variety ofdevices. The phase transfer solution in which the halometallate-cappednanocrystals are initially formed can be used as an ink. Alternatively,the halometallate-capped nanocrystals can be separated from the phasetransfer solution and then redispersed in another solution to provide ahalometallate-capped NC ink. Examples of organic solvents in which thehalometallate-capped nanocrystals can be redispersed include the organicsolvents pyridine, propylene carbonate, and N-methylformamide (NMF), andmixtures of one or more thereof with an alcohol, such as 1-propanoland/or methanol. Once the halometallate-capped NC ink has been formed,it can be coated onto the surface of a device substrate 104. The coatingprocess can be carried out using either continuous or batch methods,such as spray-coating, doctor-blading, or spin-coating, as illustratedin Example 3. The coating can then be annealed at an elevatedtemperature 106 to promote semiconductor recrystallization and graingrowth. By way of illustration only, in some embodiments of the methods,the coatings are annealed at temperatures in the range from about 300°C. to about 400° C. Annealing can be carried out, for example, using ahot plate or an infrared (IR) light source. The result is asemiconductor film composed of well-connected, sintered semiconductorgrains. If the semiconductor nanocrystals are core-shell nanocrystals,the semiconductor shells of the nanocrystals can sinter into a matrixaround the nanocrystals cores, which can be preserved intact. Forexample, core-shell nanocrystals having Group III-V semiconductor coressurrounded by Group II-VI semiconductor shells can be sintered into asemiconductor film composed of the Group III-V cores dispersed in aGroup II-VI matrix.

Some embodiments of the methods use Group II-VI semiconductornanocrystals having a metastable phase, rather than a stable phase orcore-shell semiconductor nanocrystals having a metastable phase GroupII-VI shell, rather than a stable phase Group II-VI shell. For examplethe nanocrystals, or their shells, may have a metastable wurtzite phase,rather than a stable zinc-blende phase. This is advantageous because therecrystallization and grain growth of the metastable nanocrystals ismore efficient during sintering than the recrystallization and graingrowth of the corresponding stable phase nanocrystals.

The coating 104 and annealing 106 steps can be repeated a plurality oftimes to form a film of the desired thickness. Thus, the present methodscan be used to tailor the film thickness of the semiconductor layers. Byway of illustration, semiconductor films having thicknesses of 600 nm orlower can be made. This includes semiconductor films having thicknessesof 500 nm or lower and semiconductor films having thicknesses of 300 nmor lower (e.g., in the range from 100 nm to 600 nm). Notably, themethods are able to make ultrathin films—including films of thesemiconductor having thicknesses of 200 nm or lower or even 100 nm orlower with semiconductor mean grain sizes of 100 nm or greater.

The sintered semiconductor films are highly uniform and can be producedwith a low defect density relative to semiconductor films made using ahalometallate bath treatment, such as a CdCl₂ treatment. Withoutintending to be bound to any theory of the invention, the inventorsbelieve this is because the halometallate ligands on the semiconductornanocrystals take up considerably less space than do the residuallong-chain organic ligands that are removed during a CdCl₂ treatment.Therefore, the present films undergo a smaller volume contraction duringsintering, which leads to fewer defects in the sintered films.

Once the semiconductor film has been formed, downstream processing 108can be used to incorporate the semiconductor film into a photonicdevice, such as a photovoltaic cell, or an electronic device, such as anFET. Other devices into which the semiconductor films can beincorporated as active materials include infrared photodetectors,including those in which the semiconductor film is, for example, aHg_(1-x)Cd_(x)Te film; and high energy photon detectors, such as x-rayand gamma-ray detectors, including those in which the semiconductor filmis, for example, a Cd_(1-x)Zn_(x)Te film. The basic components of thesedevices include: a first electrode comprising an electrically conductivematerial; a second electrode comprising an electrically conductivematerial; and a photoactive material or electrically active materialcomprising the semiconductor film in electrical communication with thefirst electrode and the second electrode.

The basic components of a photovoltaic cell into which the semiconductorfilms can be incorporated include: a first electrode; a secondelectrode; and a layer of photoactive material comprising thesemiconductor film disposed between and in electrical communication withthe first and second electrodes. The middle panel of FIG. 3A is aschematic diagram showing an exploded view of the layers of oneembodiment of a photovoltaic cell that can be made using the presentmethods. As described in detail in Example 2, the photovoltaic cell canbe fabricated by coating a solution of the halometallate ligand-cappedsemiconductor nanocrystals onto the surface of an electrode 302, such asan indium tin oxide (ITO) electrode and annealing the coating to sinterthe nanocrystals into a semiconductor film 304. This film provides thephotoactive layer of the photovoltaic cell, which absorbs light, such assunlight, and generates electron-hole pairs. In the embodiment of FIG.3A, ITO electrode 302 is a thin layer of ITO on the surface of a glasssubstrate 306. To finish the device, a layer of charge transportingmaterial 308, such as ZnO, is applied over the annealed film and a topelectrode 310, such as an aluminum electrode, is deposited over the ZnO(panel (d)). Although this particular example of a photovoltaic cell isillustrated with selected material layers, other materials can be used.For example, the charge transporting layer can take the form of a porousfilm of other semiconducting materials, such as titanium dioxidenanoparticles, and other metals, metal alloys, or transparent conductiveoxides can be used as electrode materials. In addition, other layerscommonly used in thin film photovoltaic cells, such as electrontransport layers, hole blocking layers, and the like, may also beincorporated into the photovoltaic cells.

Photovoltaic cells comprising light-absorbing layers formed from thesemiconductor film can have high power conversion efficiencies. Forexample, photovoltaic cells having power conversion efficiencies undersimulated full sunlight of AM 1.5G, 100 mW cm⁻² of at least 5, at least8 and at least 10% can be fabricated. These power conversionefficiencies can be obtained using very thin light-absorbing layers of600 nm or less, including light-absorbing layers having a thickness of400 nm or less. Methods for determining the power conversion efficiencyof a solar cell are provided in the Examples.

The basic components of an FET into which the semiconductor films can beincorporated include: a source electrode; a drain electrode; a gateelectrode; a conducting channel in electrical contact with the sourceelectrode and the drain electrode, the conducting channel comprising thesemiconductor film; and a gate dielectric disposed between the gateelectrode and the conducting channel. As described in detail in Example2, the FET can be fabricated by coating a solution of the halometallateligand-capped semiconductor nanocrystals onto the surface of a gatedielectric layer, such as an SiO₂ dielectric, and annealing the coatingto sinter the nanocrystals into a semiconductor film. This film providesthe conducting channel layer of the FET. The source and drain electrodescan then be formed on the conducting channel layer. The gate electrodecan be formed on the gate dielectric before or after the formation ofthe semiconductor film.

The following examples illustrate the capping of CdTe nanocrystals withCdCl₃ ⁻ ligands and HgCl₃ ⁻ ligands, the capping of HgSe nanocrystalswith CdCl₃ ⁻ ligands, the capping of HgTe nanocrystals with CdI₃ ⁻ligands, and CdTe film formations using the CdCl₃ ⁻ ligand-capped CdTenanocrystals. However, it should be understood that the methodsdescribed in the examples can also be used to make otherhalometallate-capped semiconductor nanocrystals and other semiconductorfilms, consistent with the description provided above. By way ofillustration only, the methods can be extended to the creation ofmercury cadmium telluride (MCT) thin films. For example, CdTe, HgTe, orHg_(x)Cd_(1-x)Te nanocrystals can be capped with halomercurate orhalocadmate ligands (e.g., HgCl₃ ⁻, HgBr₃ ⁻, HgI₃ ⁻, CdCl₃ ⁻, CdBr₃ ⁻,or CdI₃ ⁻) to form a colloidal solution in a polar solvent. Thissolution can then be deposited onto a substrate and annealed at 200-300°C. to form a polycrystalline MCT thin film comprising grains of thesemiconductor. This library of nanocrystal and ligand compositionsallows the composition of the resulting Hg_(x)Cd_(1-x)Te alloy to bevaried from x=0 to x=1 with a high degree of control. The resultingsemiconductor films can have bandgaps ranging from 0 eV to 1.5 eV anddevices made from these films can be used as photodetectors from thevisible through long-wave infrared (LWIR). For at least someapplications, this solution-processed approach may be preferable totraditional methods of MCT fabrication, including liquid-phase epitaxy(LPE) or molecular beam epitaxy (MBE). These epitaxial techniquesrequire extreme conditions (high temperature and/or high vacuum) andvery expensive lattice-matched single crystalline substrates. Incontrast, the halometallate-capped nanocrystal solutions can bedrop-cast on a wide variety of substrates (silicon, glass,high-temperature plastic, etc.) and annealed at mild temperatures.

EXAMPLES Example 1

This example illustrates methods of making CdCl₃ ⁻ ligand-capped CdTenanocrystals and solar cells incorporating the nanocrystals as aphotoactive material. CdTe is used here as an illustrative group II-IVmaterial. The methods described herein can also be applied to othersemiconductor nanocrystals, including other group II-IV semiconductors.

Additional details for methods of making CdCl₃ ⁻ ligand-capped CdTenanocrystals and solar cells incorporating the nanocrystals as aphotoactive material can be found in Zhang et al., J. Am. Chem. Soc.,2016, 138 (24), pp 7464-7467 and its supporting information, which areincorporated herein by reference.

Experimental Procedure

Ordered Chemicals and Purification. 99.99% trace metal cadmium oxide,cadmium chloride and ammonium chloride, 90% technical grade oleic acid(OA) and 1-octadecene (ODE), 99.5% by redistillation ethanolamine, 98%pyridinium hydrochloride, 97% tributylphosphine with isomers (TBP),99.999% indium chloride, 99.999% tellurium shot, 99%hexamethylphosphoramide (HMPA) and anhydrous hexane, toluene, ethanol,methanol, acetonitrile, pyridine (Pyr), N-methylformamide (NMF),propylene carbonate (PC), 1-propanol (1-PA) and 2-methoxyethanol werepurchased from Sigma-Aldrich. Certified ACS acetone, methanol and2-propanol (IPA) were purchased from Thermo Fisher Scientific. 10 wt %TBP:Te was prepared by dissolving 10 g Te shot in 90 g TBP overnight.ODE and OA were recrystallized by cooling the bottle in a chillerovernight to 12° and 18° C., respectively, and decanted to removeimpurities. Pyr and 1-PA were distilled to remove low and high boilingpoint impurities attributed to morphological issues in spin-coatedfilms. NMF was dried prior to use in the glovebox.

Nanocrystal Synthesis. 4.80 g CdO, 42.4 g recrystallized OA and 40.0 grecrystallized ODE were charged in a 500 mL flask and evacuatedovernight to remove trace oxygen. The flask was heated to 800° C. untilthe pressure equilibrated. Under dry nitrogen, the mixture was heated to2200° C. until the solution turned clear, indicating a completedreaction. The flask was cooled to <900° C. and evacuated. The flask washeated to 110° once the solution stopped bubbling and left until thepressure equilibrated. Under dry nitrogen, the flask was heated to 270°C. and 24 mL of 10 wt % TBP:Te was injected. The heating mantle wasremoved immediately and the flask was allowed to air cool to <50° C. Theresulting CdTe nanocrystal (NC) solution was split evenly between 8centrifuge tubes and taken into a dry nitrogen glove box. The NCs werepurified using anhydrous solvents in the glove box. Ethanol was used asa non-solvent with toluene as the solvent. The standard method of addingnon-solvent to the NC solution, precipitate using a centrifuge, decantand dissolve in solvent was utilized to achieve a high purity. Followingthe final wash, the NCs were dissolved in hexane at a concentration of˜80 mg/mL.

Halometallate Ligand Exchange. Halometallates with NH₄ ⁺ or C₅H₆N⁺cations were synthesized by mixing equimolar amount of CdCl₂ and NH₄Clor C₅H₆NCl in NMF (0.1 M). In a typical ligand exchange, 3 mL of CdTe NCsolution in hexane (˜30 mg/mL) was mixed with 3 mL of CdCl₃ solution inNMF (0.05 M). Under vigorous stirring, the NCs gradually transferredfrom hexane to NMF. Typically, it took up to 8 hours until a completephase transfer, resulting in a colorless hexane phase. The time requiredfor ligand exchange is strongly dependent on the concentration of NCs,and also affected by the cations of halometallates. A much shorter time(within 30 minutes) is required for CdTe NCs with a lower concentration(5 mg/mL). The bottom phase containing CdTe NCs was then rinsed withfresh hexane three times, followed by the flocculation of NC solution inNMF by adding a mixture of toluene (1 mL) and HMPA (0.5 mL). The NCprecipitates were collected by centrifugation, and re-dispersed in 1 mLpyridine.

The dispersion of CdCl₃ ⁻-capped CdTe NCs in pyridine was vigorouslymixed by stirring for >1 h, followed by centrifugation to remove theinsoluble part. The concentration of the stable solution can be as highas 70-80 mg/mL. In pyridine, CdCl₃ ⁻-capped NCs with C₅H₆N⁺ cationsshowed a slightly higher solubility than those with NH₄ ⁺, presumablydue to the compatibility of the cation and the solvent. Theseconcentrated solutions were deployed as soluble precursors for sinteredCdTe solar cells by spin-coating or spray-coating. The CdCl₃ ⁻-cappedNCs also disperse well in propylene carbonate (PC).

Standard Solar Cell Fabrication. For purposes of comparison, thefabrication of a solar cell using a standard approach is described here.A more detailed description is provided in Example 2. 25 mm×25 mmITO-coated glass substrates were purchased from Thin Film Devices Inc.The substrates were sonicated in deionized water (DI) and Alconoxdetergent and thoroughly rinsed with DI. The substrates were thensonicated in DI, acetone, IPA and DI individually. Following the last DIstep, the substrates were dried using nitrogen and oxygen-plasma treatedfor 10 minutes using a Harrick PDC-001 Extended Plasma Cleaner. ZnOsol-gel was prepared by sonicating 1.50 g zinc acetate dihydrate, 15 mL2-methoxyethanol, 420 μL ethanolamine and 15-45 mg InCl₃ together for 1hour and stirring overnight. The CdTe NC stock solution was purified onemore time using hexane as a non-solvent and dissolved in a 50:50 mixtureof Pyr and 1-PA to the desired concentration. The solution was sonicatedfor 10 minutes and filtered using a 0.2 μm PTFE syringe filter. Thecleaned CdTe NC solution was spin-coated onto a cleaned substrate, driedat 150° C. for 2 minutes and air cooled. The substrate was dipped in asaturated CdCl₂ bath for 15 seconds, thoroughly rinsed with IPA anddried under nitrogen flow. The substrate was annealed at 350° C. for 20seconds and allowed to cool. The process was repeated until the desiredthickness was achieved. Following the last layer, ZnO sol-gel wasspin-coated on top, and the substrate was annealed at 300° C. for 2minutes and cooled. The substrates were transferred into a glove box andheld under high vacuum overnight. The substrates were masked using ahomemade substrate holder with 8 mm² holes evenly distributed on theinterior of the substrate. 100 nm of Al and Ag each were deposited asthe top electrode. Three sides of each substrate were scratched off toexpose the ITO. Good electrical contact was established using colloidalAg paint from Ted Pella Inc.

Solar Cell Testing. The AM1.5 solar spectrum was created using a Newportmodel 67005 Xenon lamp and solar filter. The light was calibrated usinga Hamamatsu Inc, S1787-04 Si solar cell fitted into a homemade testingapparatus. The substrates were tested using a Keithley 2400 electrometercontrolled with homemade Labview software. The substrates were coveredwith an aperture mask with 6 mm² holes evenly distributed to supply acontrolled amount of light to each pixel and remove uncertainty inillumination area.

Example 2

In this example, chlorocadmate (CdCl₃ ⁻) ligand chemistry was used todesign a new CdTe NC ink for solution-processed solar cells. In thisink, CdCl₃ ⁻ ligands play a dual role: i) replace the organic ligands onCdTe NCs and afford a high solubility of NCs in a suitable solvent; andii) promote CdTe grain growth. CdTe NCs in this ink can efficientlysinter and grow into large grains. Moreover, chloride ions provideselective electronic doping of grain boundaries that helps to separatecharge carriers in CdTe solar cells.

Preparing the ink involved the exchange of insulating, long hydrocarbonligands with CdCl₃ ⁻. Spherical CdTe NCs (FIG. 2A) were synthesizedusing tetradecylphosphonic acid (TDPA) in accordance with reportedmethods (Yu, W. W.; Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15,2854). For ligand exchange, CdTe NCs dissolved in hexane were combinedwith a solution of NH₄CdCl₃ in N-methylformamide (NMF). These twoimmiscible phases were stirred until CdTe NCs transferred to NMF andformed a stable colloid. After purification, the ligand-exchanged CdTeNCs can redisperse in several solvents, including NMF, propylenecarbonate, and pyridine. NH₄CdCl₃-capped NCs were colloidally stable asCdCl₃ ⁻ (and other anions generated by self-ionization) bound to the NCsurface and provided electrostatic repulsion to overpower theinterparticle van der Waals attractive force. The binding of negativelycharged CdCl₃ ⁻ anions to NC surface was confirmed by the negativezeta-potential (−34 mV) of NH₄CdCl₃-capped CdTe NCs in propylenecarbonate. Transmission electron microscopy (TEM) images (FIG. 2B)indicated that NCs retained their size and morphology. The integrity ofCdTe NC cores after ligand exchange was also demonstrated by the opticalabsorption spectra. The absorption spectra (FIG. 2C) of CdCl₃-cappedCdTe NCs of various sizes resembled those of TDPA-capped ones. Theligand exchange procedure using NH₄CdCl₃ can be extended to CdTe NCswith other organic surfactants or morphologies (e.g., oleate-capped CdTetetrapods) and, more importantly, performed on a large scale. An inkcontaining several grams of CdCl₃ ⁻-capped CdTe NCs can be made in asingle batch. The completeness of ligand exchange on such a large scalewas monitored by Fourier transform infrared (FTIR) spectroscopy. Thevibrational peaks observed in FTIR (FIG. 1F) arising from native organicligands (e.g., 2800-3000 cm⁻¹ for C—H stretching mode, 1300-1500 cm⁻¹for C—H bending) were drastically suppressed in NCs with CdCl₃ ⁻ligands. The weak IR features for CdCl₃ ⁻-capped CdTe NCs could beassigned to solvent residue and counterions, as they disappeared aftermild annealing (200° C. under vacuum). Besides NH₄CdCl₃, CdCl₃ ⁻moieties with other cations can also behave as ligands for CdTe NCs. Forinstance, reacting an equimolar mixture of CdCl₂ and pyridiniumhydrochloride in NMF resulted in soluble species containing CdCl₃ ⁻ andC₅H₅NH⁺. CdTe NCs capped with this compound (further referred to aspyrHCdCl₃) showed similar UV-visible absorption features to those cappedwith NH₄CdCl₃. Moreover, pyrHCdCl₃ allowed a higher solubility (up to150 mg/mL) of CdTe NCs in low boiling point solvents (pyridine, itsmixture with alcohols, and nearly pure alcohols with several percentpyridine), which was crucial for the solution-based fabrication of highquality CdTe thin films.

In this ink, the surface-bound CdCl₃ ⁻ molecules play a vital role asthe grain growth promoter of CdTe NCs. CdTe NCs before and after CdCl₃ ⁻ligand exchange showed significantly different sintering behavior, asevidenced by then XRD patterns. Broad peaks (centered at ˜39°, ˜42.5°,and 46.5°) with low intensities were observed in the case ofas-synthesized CdTe NCs, corresponding to small CdTe grains (diameterabout 5 nm). The native organic ligands inhibited the sintering of NCs,even at high temperatures, as indicated by the minor changes inintensities and widths of XRD peaks. Exchanging native oleate ligandsfor pyridine did not result in a significant promotion of CdTe graingrowth upon the heat treatment. In contrast, the grain growth in CdCl₃⁻-capped CdTe NCs was considerably enhanced, as evidenced by the highintensities and narrowing of the XRD peaks starting from ˜250° C.Concomitantly, a characteristic peak for wurtzite phase (˜42.5°)disappeared, indicating a complete transition to a zinc-blende phase. Asfurther evidence of grain growth, scanning electron microscopy (SEM)images of an annealed, single-layer CdTe film deposited from CdCl₃⁻-capped CdTe NC ink revealed a uniform film of well-connected, sinteredCdTe grains. Cross-sectional SEM (FIG. 3A; left and right panels) of thecomplete CdTe device also showed a CdTe layer composed of sinteredgrains comparable in size to the thickness of the entire absorber film.

Prior to the design of CdCl₃ ⁻-capped CdTe NC ink, the use of CdTe NCswith several other inorganic surface ligands, including Cl⁻, Te²⁻, andCdTe₂ ²⁻, was attempted (Nag, A.; Kovalenko, M. V.; Lee, J.-S.; Liu, W.;Spokoyny, B.; Talapin, D. V. J. Am. Soc. Chem. 2011, 133, 10612; Zhang,H.; Jang, J.; Liu, W.; Talapin, D. V. ACS Nano 2014, 8, 7359; Norman, Z.M.; Anderson, N. C.; Owen, J. S. ACS Nano 2014, 8, 7513; Ning, Z.; Dong,H.; Zhang, Q.; Voznyy, O.; Sargent, E. H. ACS Nano 2014, 8, 10321;Fafarman, A. T.; Koh, W.-K.; Diroll, B. T.; Kim, D. K.; Ko, D.-K.; Oh,S. J.; Ye, X.; Doan-Nguyen, V.; Crump, M. R.; Reifsnyder, D. C.; Murray,C. B.; Kagan, C. R. J. Am. Soc. Chem. 2012, 133, 15753; Dolzhnikov, D.,S.; Zhang, H.; Jang, J.; Son, J. S.; Panthani, M. G.; Chattopadhyay, S.;Shibata, T.; Talapin, D. V. Science 2015, 347, 425). The simple halide,Cl⁻, failed to afford highly concentrated, stable colloidal solution ofCdTe NCs (solubility<30 mg/mL), which is a prerequisite forsolution-processed solar cells. CdTe NCs capped with Te²⁻ and CdTe₂ ²⁻sintered after annealing without CdCl₂ treatment, but had been too airsensitive. Besides, adding CdCl₂ to pyridine-exchanged CdTe NCs(referred to as the “additive approach”) was unable to remove theorganic ligands and led to poor device performance. In comparison, therobust CdCl₃ ⁻-capped CdTe NC ink in a mixture of pyridine and1-propanol can be processed in air and produced high quality CdTe layersusing a layer-by-layer approach without additional CdCl₂ treatment. Byusing a device architecture outlined in FIG. 3A (middle panel) andcurrent/light soaking (Panthani, M. G.; Kurley, J. M.; Crisp, R. W.;Dietz, T. C.; Ezzyat, T.; Luther, J. M.; Talapin, D. V. Nano Lett. 2014,14, 670), solar cells with a ˜350 nm thick CdTe absorber were fabricatedand showed a PCE of 10.3% with short-circuit current density (J_(SC)),open-circuit voltage (V_(OC)), and fill factor (FF) of ˜21 mA/cm², ˜690mV, and ˜70%, respectively (FIG. 3B and Table 1). The average valuesachieved were ˜9%, ˜20 mA/cm², ˜670 mV, and ˜68% for PCE, J_(SC),V_(OC), and FF, respectively. The photocurrent collected on each singleCdTe solar cell device (area: 8 mm²) was generated by an incident lightexposed to an area of 6 mm² using an aperture mask. The external quantumefficiency (EQE) spectrum (FIG. 3D) matched well with the measuredJ_(SC). Internal quantum efficiency (IQE) (FIG. 3D) was about 10% highercompared to EQE values, consistent with an estimate for light reflectionat the air/glass interface. These reasonably high PCE values comparedfavorably with devices with very thin CdTe layers. Traditional CdTesolar cells use several micron thick CdTe layers, which poses obviousconcerns given Te scarcity and Cd toxicity issues. Solution depositioncan be easily scaled to different film thickness as shown in FIG. 3A(left and right panels) by simply adjusting the concentration of NC inkand the number of deposited layers. For example, the solar cells with˜600 nm thick CdTe layers had a PCE of 11.6% due to improvements in bothJ_(SC) (23.7 mA/cm²) and V_(OC) (718 mV), with a FF 68% (FIGS. 3E-G andTable 1). Extensive studies of sputtered ultrathin CdTe solar cellsdemonstrated an optimized 11% PCE for a 0.5 m thick CdTe layer and 8%record efficiency for devices with a 0.25 m thick CdTe layer due toshunting and incomplete light absorption (Paudel, N. R.; Wieland, K. A.;Compaan, A. D. Sol. Energ. Mat. Sol. Cells 2012, 105, 109). Switchingfrom gas-phase to solution deposition clearly helps to improve theuniformity of ultrathin CdTe layer (FIG. 3A) resulting in 10% PCE fordevices with sub-400 nm absorber. Improvements can be achieved throughthe optimization of device stack and deposition conditions, e.g., byannealing the CdTe layers not in air but under controlled atmosphere.

TABLE 1 Summary of performance of CdTe solar cells made from the“standard” approach and the CdCl₃ ⁻-capped CdTe NC ink (spin- orspray-coated) under AM 1.5 G illumination. CdTe Absorber J_(SC)Deposition Thickness PCE V_(OC) (mA/ FF Ink Method (nm) (%) (mV) cm²)(%) CdCl₃ ⁻- Best Spin- ~350 10.3 690 21.4 70 capped coated CdTe NCTypical 9.0 670 20.0 68 Ink CdCl₃ ⁻- Best Spin- ~600 11.6 718 23.7 68capped coated CdTe NC Typical 10.0 680 22.0 67 Ink CdCl₃ ⁻- Best Spray-~350 8.8 650 21.4 63 capped coated CdTe NC Typical 8.0 650 20.0 60 Ink“Standard” Best Spin- ~400 9.6 598 22.2 72 coated Typical 8.0 550 21.0“Standard” Best Spin- ~550 12.3 684 25.8 71 coated Typical 11 650 24.569

In a series of control experiments, the performance of CdTe solar cellsmade of CdTe NCs capped with the pyrHCdCl₃ surface ligands were comparedto the same batch of NCs, but capped with pyridine. In the latter case,each spin-coated NC layer was soaked in a saturated solution of CdCl₂ inmethanol before annealing at 350° C. The latter NCs are referred to ashaving been made by the “standard approach”, which is described in moredetail in the Detailed Methods Description, below. At comparablethicknesses of CdTe absorber layers, the solar cells made with pyrHCdCl₃surface ligands showed comparable or higher PCE values, primarily due toa significant increase in V_(OC) (Table 1). This difference in V_(OC)can be ascribed to a better passivation of grain boundaries and reducedShockley-Read-Hall recombination velocity in the CdTe layer.

The ability to integrate the grain growth promoter in the form of thesurface ligands for CdTe NCs made the ink compatible with highthroughput deposition methods, such as spray-coating or doctor blading.In preliminary tests for spray-coated solar cells, a PCE (8-9%, Table 1)comparable to spin-coated devices, using CdCl₃ ⁻-capped CdTe NC ink inmethanol with a small amount of pyridine was achieved.

The CdCl₃ ⁻-capped CdSe NCs were used to fabricate field effecttransistor (FET) devices. Similar to the case of CdTe, CdSe NCs candisperse in NMF with NH₄CdCl₃ ligands without notable changes in thecrystal size. CdCl₃ ⁻ ligands promoted the grain growth of NCs withrespect to other ligands, such as I⁻. The sintered CdSe nanograins (˜20nm) showed decent electron mobility up to ˜30 cm²/Vs, depending on theannealing temperature, estimated from the FET transfer characteristics.More interestingly, the air-stable CdCl₃ ⁻ ligands allowed the ligandexchange process and FET channel fabrication, for the first time, to beperformed in air, resulting in CdSe FETs with preliminary mobilities of˜2-3 cm²/V·s.

Detailed Methods Description

1. Chemicals

Cadmium oxide (CdO, 99.99+%, trace metal basis), ammonium chloride(NH₄Cl, 99.99%), cadmium chloride (CdCl₂, 99.99%, trace metal basis),pyridinium hydrochloride (C₅H₅N.HCl, 98%), indium chloride (InCl₃,99.999%), tellurium shot (Te, 99.999%), tributylphosphine (TBP, 97% withisomers), trioctylphosphine oxide (TOPO, 99%), oleic acid (OA, technicalgrade, 90%), oleylamine (OLA, 70%), tetradecylphosphonic acid (TDPA,97%), 1-octadecene (ODE, technical grade, 90%), ethanolamine (99.5%,re-distilled), hexamethylphosphoramide (HMPA, 99%), ethanol (≥99.5%,anhydrous), toluene (≥99.8%, anhydrous), hexane (95%, anhydrous),methanol (99.8%, anhydrous), acetonitrile (99.8%, anhydrous), pyridine(99.8%, anhydrous), propylene carbonate (99.7%, anhydrous),N,N-dimethylformamide (DMF, 99.8%, anhydrous), 1-propanol (1-PA, 99.7%,anhydrous), and 2-methoxyethanol (99.9%, anhydrous) were purchased fromAldrich. N-trioctylphosphine (TOP, 97%) was purchased from Strem.Acetone (certified ACS), methanol (certified ACS), and 2-propanol (IPA,certified ACS) were purchased from Thermo Fisher Scientific. 10 wt %TBP:Te was prepared by dissolving 10 g of Te shot in 90 g of TBPovernight in a N₂-filled glove box. ODE and OA were recrystallized bycooling the bottle in a chiller overnight at 12 and 18° C.,respectively, and decanted to remove impurities. Pyridine and 1-PA weredistilled to remove low and high boiling point impurities.N-methylformamide (NMF, 99%, Alfa Aesar) and OLA were dried prior to usein glove box.

2. Nanocrystal Synthesis

Monodisperse zinc-blende CdTe nanocrystals (NCs) capped with TDPAligands were synthesized following the reported procedure (Yu, W. W.;Qu, L.; Guo, W.; Peng, X. Chem. Mater. 2003, 15, 2854). In brief, CdO(128 mg), TDPA (570 mg), and ODE (39.3 g) were evacuated at 80° C. untilequilibrated. Under dry N₂, the mixture was heated to 300° C. until allpowders dissolved. At this temperature, a solution containing 2.5 g of10 wt. % TBP:Te, 2.5 g of TBP, and 15 g of ODE was swiftly injected. Thereaction mixture immediately turned green and then orange within 30seconds. Aliquots were taken at 1, 3, 4, and 5 minutes after injectionand were quenched by adding toluene. The resulting CdTe NCs werepurified using anhydrous solvents in the glove box. NCs wereprecipitated by a 1:1 mixture of anhydrous methanol and 1-propanol, andredispersed in toluene. Prior to the ligand exchange, NCs wereprecipitated with ethanol and redispersed in hexane.

Wurtzite CdTe NCs capped with oleate were synthesized with a modifiedmethod described in Jasieniak, J.; MacDonald, B. I.; Watkins, S. E.;Mulvaney, P. Nano Lett. 2011, 11, 2856; and MacDonald, B. I.;Gengenbach, T. R.; Watkins, S. E.; Mulvaney, P.; Jasieniak, J. J. ThinSolid Films 2014, 558, 365. CdO (4.80 g), OA (42.4 g, recrystallized),and ODE (40.0 g, recrystallized) was loaded in a 500 mL three-neckedflask and evacuated overnight to remove trace oxygen. Afterward, theflask was heated to 80° C. until the pressure equilibrated. Under dryN₂, the mixture was heated to 220° C. until the solution turned clear,indicating a completed reaction to form cadmium oleate (Cd(OA)₂). Theflask was cooled and dried under vacuum at 110° C. to remove watergenerated by the reaction. Under dry N₂, the flask was heated to 270°C., followed by the quick injection of 24 mL of 10 wt % TBP:Te.Immediately after the injection, the heating mantle was removed and theflask was quickly cooled to room temperature. The resulting CdTe NCswere purified using anhydrous solvents in the glove box. Ethanol wasused as the non-solvent while toluene as the solvent. After severalprecipitation-redispersion cycles with ethanol/toluene, the purified NCswere dissolved in hexane at a concentration of ˜80 mg/mL.

Wurtzite CdSe NCs capped with OA were synthesized using Cd(OA)₂ as theCd precursor. In brief, 1.2 g of TOPO, 2.25 mL of 1.0 M Cd(OA)₂ solutionin OA, and 12 mL of ODE were loaded in a 100 mL three-necked flask anddried under vacuum at 70° C. for 1 hour. Afterward, the solution washeated to 300° C. under N₂. A stock solution containing 4 mL of 1.0 MTOPSe solution in TOP and 3 mL of OLA was swiftly injected at 300° C.The mixture was kept at 280° C. for 2-3 minutes and quickly cooled toroom temperature. The CdSe NCs can be isolated by adding ethanol to thecrude solution followed by centrifugation. CdSe NC precipitates canredisperse in non-polar solvents (e.g., hexane). The washing withethanol/hexane as non-solvent/solvent was repeated several cycles toremove excess organic ligands. Finally, the purified CdSe NCs weredissolved in hexane.

3. Preparation of CdTe NC Inks

a) Pyridine-exchanged CdTe NC Ink

To prepare a soluble CdTe NC ink for fabricating CdTe thin films usingthe “standard” or “additive” approach, CdTe NC solution in toluene wasprecipitated with ethanol and redispersed in anhydrous pyridine at aconcentration of ˜80 mg/mL. The solution of CdTe NCs in pyridine wasstirred under N₂ overnight on a hotplate set to 100° C., followed byprecipitation using hexane. The CdTe NC precipitates were redispersed infresh pyridine to prepare the “pyridine-exchanged” CdTe NC ink for the“standard” or “additive” approach.

b) Ligand Exchange of CdTe NCs with CdCl₃ ⁻ Ligands and Preparation ofthe CdCl₃ ⁻-capped CdTe NC Ink

Chlorocadmates (CdCl₃ ⁻) with NH₄ ⁺ or C₅H₅NH⁺ cations were synthesizedby mixing equimolar amount of CdCl₂ and NH₄Cl or C₅H₅N.HCl in NMF (0.1M). In a typical ligand exchange, 3 mL of oleate-capped CdTe NC solutionin hexane (˜30 mg/mL) was mixed with 3 mL of CdCl₃ ⁻ solution in NMF(0.1 M). Under vigorous stirring, NCs gradually transferred from hexaneto NMF. Typically, it took up to several hours until a complete phasetransfer, resulting in a colorless hexane phase. The time required forligand exchange was strongly dependent on the concentration of NCs, andalso affected by the cations of chlorocadmates. A much shorter time(within 15 minutes) is required for CdTe NCs with a lower concentration(5 mg/mL). The bottom phase containing CdTe NCs was then rinsed withfresh hexane three times. In detail, 3 mL of fresh hexane was mixed withthe solution of CdCl₃ ⁻-capped CdTe NCs in NMF, forming a two-phasemixture. This mixture was vigorously stirred for about 20 minutes.During this process, residual organic ligands and related speciessoluble in the non-polar hexane phase were removed from the NC solutionin NMF. The hexane layer was then discarded and replaced with freshhexane. After a triple wash with hexane, a mixture of toluene (1 mL) andHMPA (0.5 mL) was added, leading to the flocculation of NC solution. TheNC precipitates were collected by centrifugation, and re-dispersed in 1mL of propylene carbonate or pyridine. The ligand exchange procedure canbe scaled up to produce >1 g of CdCl₃ ⁻-capped CdTe NCs in a singlebatch. The solution of CdCl₃ ⁻-capped CdTe NCs in pyridine wasvigorously stirred for ˜1 hour in air, followed by centrifugation toremove the insoluble part. The concentration of the colloidally stablesolution (the “CdCl₃ ⁻-capped CdTe NC ink”) can be as high as 150 mg/mL.In pyridine, CdCl₃ ⁻-capped NCs with C₅H₅NH⁺ cations showed a slightlyhigher solubility than those with NH₄ ⁺, presumably due to thecompatibility of the cation and the solvent.

4. Characterization Techniques

Transmission electron microscopy (TEM) images of NCs were obtained usinga 300 kV FEI Tecnai F30 microscope. The optical absorption spectra of NCsolutions were collected using a Cary 5000 UV-Vis-NIR spectrophotometerin the transmission mode. To investigate the evolution of size andexcitonic features of CdCl₃ ⁻-capped CdSe NCs, thin films were preparedby drop-casting NC solution in NMF on quartz substrates, followed bydrying under vacuum to remove the solvent. The dried CdCl₃ ⁻-capped CdSeNC thin films were annealed at various temperatures on a hot-plate in aN₂-filled glove box. The absorption spectra of thin films were measuredin the transmission mode. Fourier-transform infrared (FTIR) spectra wereacquired in the transmission mode using a Nicolet Nexus-670 FTIRspectrometer. Samples for FTIR measurements were prepared by dropcasting concentrated NC dispersions on KBr crystal substrates(International Crystal Laboratories) and were then dried under vacuum toremove solvent molecules. Additional annealing at 200° C. under vacuumwas applied to the CdCl₃ ⁻-capped NC samples to completely remove anyresidual solvents. IR absorbance was normalized to the weight ofabsorbing material deposited per unit area of the substrate. Toquantitatively compare IR spectra, standard background subtraction andbaseline correction routines were applied. Scanning electron microscopy(SEM) images of sintered CdTe thin films and the complete CdTe solarcell devices were acquired on FEI NanoSEM Nova 200 (top-view), FEINanoSEM Nova 630 and Zeiss-Merlin (cross-sectional), respectively. Fortop-view SEM, single-layer CdTe thin films were deposited on siliconsubstrate from the new CdCl₃ ⁻-capped CdTe NC ink (annealed at 350° C.for 20 s). The same CdTe thin films were used for wide angle powderX-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS)measurements. The XRD patterns of CdTe thin films made from organicallycapped CdTe NCs and the new ink were collected using a Bruker D8diffractometer with a Cu K_(α) X-ray source operating at 40 kV and 40mA. In-situ XRD measurements were carried out by ramping the temperaturefrom 25 to 600° C. (3° C./min) with frames taken every 195 sec. The thinfilm samples were enclosed in an O-ring sealed dome with a plastic capand a temperature-controllable metallic bottom. The dome was evacuatedand re-filled with N₂ several cycles to remove residual air or moistureprior to the measurement. Afterward, the dome was kept under vacuumduring the in-situ measurement. The samples were annealed by thetemperature-controllable bottom part of the dome during the measurement.The source and detector of the diffractometer were set to 190 withrespect to horizontal. A 2D intensity color map was made by compilingthe frames using a homemade Matlab software. XPS analysis on sinteredCdTe thin films made from “standard,” “additive” (see details below),and the new CdCl₃ ⁻-capped CdTe NC ink was performed on a Kratos AXISNova spectrometer using a monochromatic Al K_(α) source (hν=1486.6 eV).The Al anode was powered at 10 kV and 15 mA. Instrument base pressurewas 1×10⁻⁹ Torr. High-resolution spectra in Cd 3d, Te 3d, C is, and Cl2p regions were collected using an analysis area of 0.3×0.7 mm² and 20eV pass energy. All spectra were background subtracted using XPSsubtraction software in Origin. Te and Cd spectra were subtracted usinga Tougaard algorithm while C and Cl used a Shirley algorithm. The XPSintensities for all elements were normalized to the area under the Cd 3dcurves for proper comparison. Zeta-potential (ζ-potential) data werecollected using a Zetasizer Nano-ZS (Malvern Instruments, UK).

5. CdTe Solar Cell Fabrication

a) The “Standard” Approach

In the “standard” approach, the CdTe absorber layer was spin-coated frompyridine-exchanged CdTe NC ink through a layer-by-layer depositionapproach, together with interlayer chemical (CdCl₂) and thermaltreatment (350° C., 20 s). In detail, 25 mm×25 mm indium tin oxide(ITO)-coated glass substrates (Thin Film Devices Inc) were cleaned bysequential sonication in deionized water (DI) and Alconox detergent, DI,acetone, IPA, and DI. Afterward, the substrates were dried under N₂, andhydrophilized for 10 minutes using a Harrick PDC-001 Extended PlasmaCleaner. The pyridine-exchanged CdTe NC solution was precipitated byhexane and dissolved in a 1:1 mixture of pyridine and 1-PA at thedesired concentration. The solution was sonicated for 10 minutes andfiltered through a 0.2 μm PTFE syringe filter. The filtered CdTe NCsolution was spin-coated onto an ITO substrate at 800 rpm for 30 secondsfollowed by 2000 rpm for 10 s, dried at 150° C. for 2 min, and cooled inair. For the CdCl₂ treatment, the spin-coated CdTe layer was dipped in asaturated CdCl₂ bath in methanol for 15 seconds, thoroughly rinsed withIPA and dried under N₂ flow. The substrate was annealed at 350° C. for20 seconds and cooled in air. The whole process (spin-coating, CdCl₂treatment, thermal treatment) was repeated multiple times (12-20) untilthe desired thickness was achieved. Using this approach, devices with˜400 or ˜550 nm-thick CdTe active layers were fabricated by spin-coatingdifferent layers of CdTe NC ink.

The ZnO layer was deposited on top of CdTe by spin-coating 300 μL of ZnOsol-gel at 3000 rpm for 30 s, followed by annealing at 300° C. for 2minutes. The ZnO sol-gel was prepared by sonicating a mixture of 1.50 gof zinc acetate dihydrate, 15 mL of 2-methoxyethanol, 420 μL ofethanolamine, and 15-45 mg of InCl₃ for 1 hour, and subsequently stirredovernight.

After deposition of the ZnO layer, the substrates were transferred intoa glove box and kept under high vacuum overnight. Top Al contacts (100nm) were deposited by thermal evaporation through a homemade maskfeaturing evenly distributed 8 mm² holes. Ag (100 nm) was deposited ontop of Al to increase device longevity. Three sides of the device stackwere scratched off to expose the ITO. Electrical contact was establishedusing colloidal Ag paint (Ted Pella Inc).

For XPS studies on thin films made from the “standard” approach, asingle layer of CdTe was deposited on ITO or silicon substrate, followedby CdCl₂ treatment and annealing.

b) The “Additive” Approach

In the “additive” approach, about 5 wt. % of CdCl₂ (with respect to theamount of CdTe NCs) was added to the pyridine-exchanged CdTe NC ink. Themixture was used as the soluble precursor for CdTe layers. Solar celldevice fabrication was performed in a similar manner with the “standard”approach without the interlayer CdCl₂ treatment. The solar cell devicemade from the “additive” approach showed poor performance, as shown inFIG. 4. For the XPS study, a single layer of CdTe was deposited on ITOfrom the mixture of CdCl₂ and pyridine-exchanged CdTe NC ink, followedby annealing at 350° C. for 20 seconds.

c) The New CdCl₃ ⁻-capped CdTe NCs Ink Approach

A similar, but CdCl₂ treatment-free approach was adopted in thefabrication of CdTe adsorber layer from the designed CdCl₃ ⁻-capped CdTeNCs ink. In brief, the CdCl₃-capped CdTe NC solution in pyridine wasdiluted with 1-propanol in a 1:1 volume ratio, forming an ink of about40 mg/mL. Layers of CdTe were spin-coated on an ITO substrate withinterlayer thermal treatment (350° C., 20 s) for CdTe grain growth. ForXRD, top-view SEM, and XPS studies on CdTe thin films from the new ink,a single layer of CdTe was deposited on ITO or silicon substrate,followed by annealing.

6. CdTe Solar Cell Device Characterization

Devices were tested under the illumination of an Xe lamp with a AM 1.5Gfilter (Newport 67005) and calibrated with a Si photodiode with a KG5filter (Hamamatsu Inc, S1787-04). The illumination area was controlledby a self-aligning stainless steel aperture mask with evenlydistributed, nominally 6 mm² circular holes (5.94 mm² measured). JVcharacteristics were acquired using a Keithley 2400 source metercontrolled by a Labview interface. To mitigate heating duringmeasurements, the perimeter of the cell was in direct contact to an Alheat sink. The instruments were controlled and data collected using ahomemade Labview program. Current/Light soaking was done by applying 2-3V (forward bias) to the device under illumination for 5 minutes (7minutes for devices made from the “standard” approach with a ˜550nm-thick CdTe layer). Typically, this generated a current density of˜2.5 A cm⁻². The current was monitored carefully to not exceed a 3 Acm⁻², as current densities greater than this generally causedperformance degradation. Holding the devices in reverse bias generallycaused a transient decrease in performance (due to reduced V_(OC)).External quantum efficiency (EQE) and internal quantum efficiency (IQE)measurements were taken using Oriel IQE-200 with a step of 10 nm for thewavelength. Capacitance-voltage (Mott-Schottky) data were acquired usinga Gamry Reference 600 potentiostat. Data were acquired using a frequencyof 500 Hz with an amplitude and step size of 5 and 10 mV, respectively.To compare performance of devices made from the “standard” approach andthe new CdCl₃ ⁻-capped CdTe NC ink, the same batch of oleate-capped CdTeNCs were used. Pyridine-exchange and ligand exchange with CdCl₃ ⁻ werecarried out on NC for the “standard” and the new ink, respectively.Solar cells with ˜400 and ˜600 nm CdTe active layers were fabricated byusing both approaches, with all other procedures (e.g., treatment of ITOsubstrates, the deposition of ZnO and Al/Ag layers) remaining identical.

7. Field-effect-transistor (FET) Device Fabrication and ElectricalMeasurements

Prime grade, n-type arsenic doped Si wafers with 100 nm-thick SiO₂ gatedielectrics (<0.005 Ωcm, NamKang High-Tech) were used as gatesubstrates. The substrates were cleaned by piranha treatment followed bymultiple rinses with DI water before use. Solutions of CdCl₃ ⁻-cappedCdSe NCs (˜70 mg/ml in NMF) were spin-coated on the cleaned substratesat 600 rpm for 6 seconds, followed by 2000 rpm for 60 seconds. A 500 Winfrared lamp was used to maintain the temperature of solutions andsubstrates at ˜70° C. The resulting 30-40 nm-thick NC films wereannealed at 100° C. for 1 hour to evaporate residual solvents, and thenannealed at 200, 250, 300, or 350° C. for 30 minutes. 100-nm-thick Alsource/drain electrodes were thermally evaporated through a shadow maskto complete a top-contact, bottom-gate FET structure (channel width W:1500 μm and channel length L=125 μm). All fabrication steps exceptsubstrate cleaning were performed in a N₂-filled glove box. The ligandexchange of CdSe NCs with NH₄CdCl₃ and deposition of the FET channel inair was also attempted. In this case, the spin-coating of CdCl₃ ⁻-cappedCdSe NCs on gate substrates was performed in air, followed by thethermal annealing at 300° C. for 30 minutes. The source/drain electrodeswere evaporated in a N₂-filled glove box.

FET devices were measured using a semiconductor analyzer (Agilent B1500A) in a N₂-filled glovebox. The linear regime FET mobility wascalculated by fitting the experimental data to the following equation:μ=(L/WC_(i)V_(DS))(dI_(DS)/dV_(G)) for the linear regime, where C_(i),V_(DS), I_(D), and V_(G) are the capacitance per unit area, drain-sourcevoltage, drain current, and gate voltage, respectively.

For proper comparison of concentrations of elements in single-layer CdTethin films made from different approaches, the XPS intensities for Te3d, C 1s, and Cl 2p were normalized by the area under the Cd 3d curves.The normalized Cd 3d curves for thin films made from the “standard”approach and new CdCl₃ ⁻-capped CdTe NC ink were similar, while thosefor the sample made from “additive” approach showed shoulder peaks athigher binding energies. These components should be attributed toresidual CdCl₂ since a large amount of CdCl₂ (5 wt % with respect toCdTe) was introduced to the pyridine-exchange CdTe NCs for the“additive” approach. As expected, similar intensities were observed inTe 3d spectra for different samples. Thin film made from the new inkshowed two peaks at higher binding energies (˜585 and ˜576 eV for Te3d_(3/2) and 3d_(5/2), respectively), probably due to the formation oftellurium oxides at the surface. Importantly, thin films made from the“standard” approach and the new ink showed much reduced C 1s intensitiescompared to the “additive” sample. This reduction in carbon content, orthe amount of residual organic ligands, is critical to achievingefficient charge separation and transport, which govern the overallpower conversion efficiency (PCE). As a consequence, solar cells madefrom the “additive” approach, with a significantly higher amount ofresidual organic ligands, suffered from poor device performance. XPSconfirmed the existence of Cl in all samples. According to previousresearch, the CdTe sintering/recrystallization occurs with the formationof Te—Cl compounds (TeCl₂ or TeCl₄) in a liquid or gas phase, whichenhance the mobility of nearby Cd and Te atoms, resulting inwell-sintered CdTe grains.⁴ In the new ink, the surface CdCl₃ ⁻ ligandsprovide adequate Cl⁻ for CdTe NC sintering.

Example 3

This example describes a method for combining roll-to-roll friendlytechniques into a single, integrated process for solar cell fabrication.CdCl₃ ⁻-capped CdTe NCs were easily deposited via spray-coating ordoctor-blading by adjusting the procedure outlined in Example 2 toimprove solubility and increase film thickness for a single layer.Infrared (IR) lamps, rather than a hot plate, were used to anneal thesubstrates during layer-by-layer deposition. All of these advances inCdTe absorber layer deposition seamlessly integrate into a single,roll-to-roll friendly process that can be adapted to different substrategeometries and materials.

Experimental Procedure

1. Chemicals

Cadmium oxide (CdO, 99.99+%), tellurium shot (Te, 99.999%), cadmiumchloride (CdCl₂, 99.99%), pyridinium hydrochloride (pyr.HCl, 98%), zincacetate dihydrate (Zn(OAc)₂.2H₂O, 99.999%), indium chloride (InCl₃,99.999%), oleic acid (OA, technical grade, 90%), 1-octadecene (ODE,technical grade, 90%), tributylphosphine (TBP, 97% with isomers),toluene (≥99.8%, anhydrous), ethanol (≥99.5%, anhydrous), pyridine(99.8%, anhydrous), hexane (95%, anhydrous), N-methylformamide (NMF,99%), hexamethylphosphoramide (HMPA, 99%), 1-propanol (1-PA, 99.7%,anhydrous), 2-methoxyethanol (99.9%, anhydrous), and ethanolamine(99.5%, redistilled) were purchased from Aldrich. Molecular sieves grade564 (Type 3A, 8-12 mesh beads), acetone (certified ACS), methanol(certified ACS), and 2-propanol (IPA, certified ACS) were purchased fromThermo Fisher Scientific. Aluminum (Al, 99.99%) and silver (Ag, 99.99%)pellets were purchased from Kurt J. Lesker Company. PELCO colloidalsilver paste (Ag paste) was purchased from Ted Pella, Inc. 10 wt. %TBP:Te was prepared by dissolving 10 g of Te shot in 90 g of TBPovernight in a N₂-filled glove box. ODE was recrystallized by coolingthe bottle in a chiller overnight at 12° C. and decanted to removeimpurities. OA was cooled to 16° C. overnight and vacuum filtered toremove high melting point impurities. Pyridine, NMF, and 1-PA weredistilled to remove low and high boiling point impurities. NMF and HMPAwere dried over molecular sieves and filtered with 0.2 μm PTFE filterbefore use.

2. CdTe NC Ink Preparation.

a) Oleate-capped CdTe NC Synthesis

CdTe NCs capped with oleate were synthesized with a modified methoddescribed by MacDonald et al. (Jasieniak, J.; MacDonald, B. I.; Watkins,S. E.; Mulvaney, P. Nano Lett. 2011, 11, 2856; MacDonald, B. I.;Gaspera, E. D.; Watkins, S. E.; Mulvaney, P.; Jasieniak, J. J. J. Appl.Phys. 2014, 115, 184501.) In short, 4.80 g CdO, 42.4 g recrystallizedOA, and 40.0 g recrystallized ODE were charged in a 500 mL flask andevacuated overnight to remove trace oxygen. The flask was heated to 80°C. until the pressure equilibrated. Under dry nitrogen, the mixture washeated to 220° C. until the solution turned clear, indicating acompleted reaction. The flask was cooled to <90° C. and evacuated. Theflask was heated to 110° C. once the solution stopped bubbling and leftuntil the pressure equilibrated. Under dry nitrogen, the flask washeated to 270° C. and 24 mL of 10 wt. % TBP:Te was injected. The heatingmantle was removed immediately and the flask was allowed to air cool to<50° C. The resulting CdTe NC solution was split evenly and purifiedusing anhydrous toluene and ethanol as the solvent/non-solventcombination.

b) Pyridine Ligand Exchange and Pyridine-capped CdTe NC Ink

Following 4-6 purification cycles, CdTe NCs were redispersed inanhydrous pyridine at a concentration of ˜80 mg/mL. The solution wasstirred under N₂ overnight on a hotplate set to 100° C., followed byprecipitation using hexane. The CdTe NC precipitates were re-dispersedin fresh pyridine to prepare the pyridine-capped CdTe NC stock solution.The stock pyridine-capped CdTe NC solution was precipitated by hexaneand dissolved in a 1:1 mixture of pyridine and 1-PA to the desiredconcentration. The solution was sonicated for 10 minutes and filteredthrough a 0.2 μm PTFE syringe filter to prepare the spin-coatingsolution.

c) CdCl₃ ⁻ Ligand Exchange

The procedure was adapted from the process described previously inExample 2. In short, chlorocadmates (CdCl₃ ⁻) with pyridinium (pyr-H⁺)cations were synthesized by mixing equimolar amount of CdCl₂ and pyr.HClin NMF (0.1 M). In a typical ligand exchange, 18 mL of the oleate-cappedCdTe NC solution in hexane (˜30 mg/mL) was mixed with 18 mL of CdCl₃ ⁻solution in NMF (0.1 M). Under vigorous stirring, NCs graduallytransferred from hexane to NMF. Upon phase transfer, the bottom phasecontaining CdTe NCs was then rinsed with fresh hexane three times.

i. “Original” CdCl₃ ⁻-capped CdTe NC Ink.

Following the CdCl₃ ⁻ ligand exchange (Section 2c), a mixture of toluene(6 mL) and HMPA (3 mL) was added, leading to the flocculation of NCs insolution. The NC precipitates were collected by centrifugation, andre-dispersed in 5 mL of pyridine. The solution of CdCl₃ ⁻-capped CdTeNCs in pyridine was vigorously stirred for ˜2 hours in air, followed bycentrifugation to remove the insoluble part. An equal amount of 1-PA wasadded to the NC solution in pyridine to make the “original” ink.

ii. “Overwashed” CdCl₃ ⁻-capped CdTe NC Ink

Following CdCl₃ ⁻ ligand exchange (Section 2c), the NCs wereprecipitated with the same non-solvent mixture outlined previously(Section 2c.i). However, instead of re-dispersing in pyridine, the NCswere dissolved in NMF (˜18 mL). The same precipitation and re-dispersingprocedure was repeated. Following a third precipitation, the NCs werere-dispersed in 2.5 mL pyridine and stirred vigorously for ˜2 hrs. Thesolution was centrifuged to remove insoluble NCs and a concentration wascalculated. Additional pyr-HCdCl₃ ligand solution in pyridine was addedto the NC solution in varying amounts to replenish Cl necessary forgrain growth. An equal amount of 1-PA was added to the solution to makethe “overwashed” ink.

3. CdTe Absorber Layer Deposition and Treatments

a) Substrate Preparation

In detail, 25 mm×25 mm indium tin oxide (ITO)-coated glass substrates(Thin Film Devices Inc.) were cleaned by sequential sonication indeionized water (DI) and Alconox detergent, DI, acetone, IPA, and DI.Afterward, the substrates were dried under N₂, and hydrophilized for 10minutes using a Harrick PDC-001 Extended Plasma Cleaner.

b) Deposition of CdTe NC Ink.

i. Spin-coating

The CdTe NC inks outlined previously (Section 2b, c.i, & c.ii) werespin-coated using the following procedure. Using the freshly plasmatreated ITO substrates, the CdTe NC ink was pipetted (˜250 μL) onto thesubstrate and spun at 800 rpm for 30 seconds followed by 2000 rpm for 10seconds. The substrate was transferred to a hot plate and dried at 150°C. for 2 minutes.

ii. Spray-coating

The CdTe NC ink outlined previously (Section 2b, c.i, & c.ii) wasdiluted with methanol by 6 parts methanol to 1 part NC solution. Thelayer thickness was controlled by changing the NC concentration in 1:1pyridine: 1-PA. A homemade spray-coating system was built by using a hotplate and a Paasche airbrush set to 45° to create a thinner wettinglayer. The ink was loaded into the airbrush and sprayed briefly to wetthe surface. The spray was controlled by solenoid valves attached to apower supply. The substrate was heated to 38° C. to facilitate drying.The spray-coating system was upgraded by making a metal turntable heatedto 38° C. to move the substrates through the spray and process multiplesubstrates at a time. The spray nozzle was upgraded to a VMAU-316SSspraying assembly from Spraying Systems Co. to more easily adjust sprayparameters. Upon deposition, the substrate was dried at 150° C. for 2minutes.

iii. Doctor-blading

The same ink preparation procedure outlined for spray-coating (Section3b.ii) was used. An Al block was heated on a hot plate to 40° C. tofacilitate smooth deposition. Glass slides were placed onto the block toact as height guides. A small amount of the NC solution (˜75 μL) waspipetted onto the substrate and a glass rod was used to smooth the filmby moving back and forth. The excess was wicked away by sweeping the rodonto the glass slides. Upon deposition, the substrate was dried at 150°C. for 2 minutes.

c) Chemical and Thermal Treatment.

i. CdCl₂ Bath and Annealing for CdTe/Pyridine

For the CdCl₂ treatment, the substrate was cooled in air and was dippedinto a saturated CdCl₂ bath in methanol at ˜60° C. for 15 s, thoroughlyrinsed with IPA and dried under N₂ flow. The substrate was annealed at350° C. on a hot plate (or under an IR lamp shielded with Al foil) for20 seconds and cooled in air. The whole process (deposition, drying,CdCl₂ treatment, thermal treatment) was repeated multiple times (12-20)until the desired thickness was achieved.

ii. Annealing Only

For CdCl₃ ⁻-capped CdTe NC inks, there was no need for a CdCl₂ bathtreatment. Instead, the substrate was transferred directly from thedrying plate to the annealing plate. The substrate was annealed at 350°C. on a hot plate (or under an IR lamp shielded with Al foil) for 20seconds and cooled in air. The whole process (deposition, drying,annealing) was repeated multiple times (12-20) until the desiredthickness was achieved.

d) Spray-coating onto Curved Substrates

Glass rods, beads, and plano-convex lenses were purchased from variousoutside vendors. They were affixed to the spray-coater using doublesided tape. For full devices, special holders would be advantageous toassure consistency as well as reduce mistakes from processingdifficulties.

4. Finishing CdTe Solar Cell

a) ZnO n-type Layer

The ZnO layer was deposited on top of CdTe by spin-coating 300 μL of ZnOsol-gel at 3000 rpm for 30 seconds, followed by annealing at 300° C. for2 minutes. The ZnO sol-gel was prepared by sonicating a mixture of 1.50g of Zn(OAc)₂ 2H₂O, 15 mL of 2-methoxyethanol, 420 μL of ethanolamine,and 15-45 mg of InCl₃ for 1 h, and subsequently stirred overnight.

b) Electrode Deposition

The substrates were transferred into a glove box and kept under highvacuum (˜10⁻⁹ Torr) overnight. Top Al contacts (100 nm) were depositedby thermal evaporation through a homemade mask, featuring evenlydistributed 8 mm² holes. Ag (100 nm) was deposited on top of Al toincrease device longevity. Three sides of the device stack werescratched off to expose the ITO. Electrical contact was establishedusing Ag paint.

5. Characterization Techniques

The optical absorption spectra of NC solutions were collected using aCary 5000 UV-Vis-NIR spectrophotometer in the transmission mode.Scanning electron microscopy (SEM) images of the complete CdTe solarcell devices were acquired on Zeiss-Merlin. X-ray photoelectronspectroscopy (XPS) analysis was performed on a Kratos AXIS Novaspectrometer using a monochromatic Al K_(α) source (hν=1486.6 eV). TheAl anode was powered at 10 kV and 15 mA. Instrument base pressure was1×10⁻⁹ Torr. High-resolution spectra in Cd 3d, Te 3d, C is, Cl 2p and P2p regions were collected using an analysis area of 0.3×0.7 mm² and 20eV pass energy.

6. Photovoltaic Characterization

Devices were tested under the illumination of a Xe lamp with an AM 1.5Gfilter (Newport 67005) and calibrated with a Si photodiode with a KG5filter (Hamamatsu Inc, S1787-04). The illumination area was controlledby a self-aligning stainless steel aperture mask with evenlydistributed, nominally 6 mm² circular holes (5.94 mm² measured). Currentdensity versus voltage (JV) curves were acquired using a Keithley 2400source meter controlled by a Labview interface. To mitigate heatingduring measurements, the perimeter of the cell was in direct contact toan Al heat sink. The instruments were controlled and data collectedusing a homemade Labview program. Current/light soaking was done byapplying 2-3 V (forward bias) to the device under illumination forvarying amounts of time. Typically, this generated a current density of˜2.5 A cm⁻². The current was monitored carefully to not exceed a 3 Acm⁻², as current densities greater than this generally causedperformance degradation. Holding the devices in reverse bias generallycaused a transient decrease in performance (due to reduced V_(OC)).External quantum efficiency (EQE) measurements were taken using OrielIQE-200 with a step of 20 nm for the wavelength. Capacitance-voltage(Mott-Schottky) data were acquired using a Gamry Reference 600potentiostat. Data were acquired using a frequency of 500 Hz with anamplitude and step size of 5 and 10 mV, respectively.

Results and Discussion

A procedure that combines doctor-blading (or spray-coating), CdCl₃ ⁻chemistry, and IR lamps integrated into a roll-to-roll friendly processwas developed. Transitioning to spray-coating or doctor-blading allowsfor a continuous process stream without the need to load the substrateonto a platform that grips it. CdCl₃ ⁻ chemistry eliminates the need forthe CdCl₂ treatment, a tedious process that is difficult to reproduce.Instead, the ink is self-contained, comprising the CdTe NCs and theCdCl₂ grain growth promoter. Changing from a hot plate to an IR lampproves the substrate does not need to be heated from the glass side tocreate continuous grains of CdTe throughout the film. IR lamps allow thesubstrate to simply move through a heated zone instead of requiring ahot surface.

Spin-coating is not conducive to roll-to-roll fabrication because itscales poorly to large areas, wastes considerable amounts of material,is not a high-throughput deposition method, and limits the geometry toplanar substrates. All of these factors make other deposition methods,such as spray-coating or doctor-blading, better alternatives. Thedevices made via spray-coating and doctor-blading had roughly the sameefficiencies as those made using spin-coating, as illustrated in the JVcurves of FIG. 5.

The devices made using roll-to-roll friendly techniques were comparableto spin-coated devices. Both techniques proved effective at consistentlyimproving V_(oc), a device parameter that typically depends more ondevice architecture than processing conditions. This demonstrates thatthe CdTe absorber layer is denser with fewer defects that contribute toShockley-Read-Hall recombination and increase shunting. J_(sc) showed asmuch variability as is typical for the CdTe NC ink.

Spin-coating was a materials inefficient method for depositing the ink.200 μL of 40 mg/mL NC ink was required to cover a single 25 mm×25 mmsubstrate. Over the course of 20 layers, ˜500 nm of CdTe was depositedonto the substrate. This means about 160 mg of NC is required to deposit˜1.9 mg of CdTe, conservatively. The result was that only ˜1% of thematerial remains on the substrate during spin-coating. For the samethickness device, 80 mg of CdTe NC is required during spray-coating andonly 2 mg is required for doctor-blading. Unlike spin-coating,spray-coating can accommodate larger substrates without significantchange to the film consistency. Larger areas also become more materialsefficient. These techniques were both attempted without the benefit ofprocess engineers optimizing it.

Example 4

This example describes the formation of CdCl₃ ⁻-capped HgSe nanocrystalsand CdI₃ ⁻-capped HgTe nanocrystals.

HgSe Nanocrystal Synthesis. HgSe nanocrystals were synthesized accordingto the methods described in Z. Deng et al. ACS Nano, 2014, 8, 11707,scaled up by a factor of two. Briefly, 0.2 mmol HgCl₂ (54 mg) was addedto 8 mL oleylamine and degassed at 110° C. for an hour. This solutionwas put under nitrogen and a solution of 0.2 mmol selenourea (25.2 mg)in 2 mL oleylamine was injected. The solution, which immediately turnedblack, was heated at 110° C. for 10 minutes. The reaction was stopped bythe removal of the heat and the injection of 15 mL tetrachloroethylene(TCE). The HgSe nanocrystals were isolated by the addition of methanolnon-solvent and centrifugation. The resulting nanocrystal solid wasdissolved in 3 mL TCE.

CdCl₃ ⁻ Capping of HgSe Nanocrystals. The nanocrystals were isolatedfrom the TCE by the addition of ethanol non-solvent and centrifugationand then redispersed in hexane. For ligand exchange, 1 mL of the HgSenanocrystal solution (˜25 mg/mL) was layered atop 1 mL of 0.1 M NH₄CdCl₃or (pyridinium)CdCl₃ in NMF and stirred for two hours. The nanocrystalstransferred to the polar phase, and the non-polar phase was washed with1.5 mL hexane four times. The nanocrystal suspension in NMF was washedwith a solution of 200 μL toluene and 100 μL HMPA, isolated bycentrifugation, and redispersed in 1 mL NMF.

HgTe Nanocrystal Synthesis. HgTe nanocrystals were synthesized accordingto the methods described in S. E. Keuleyan et al. ACS Nano, 2014, 8,8676. Briefly, 0.2 mmol HgCl₂ (54 mg) was dissolved in 6 goctadecylamine at 120° C., and this solution was degassed at 120° C. foran hour. This solution was put under nitrogen and cooled to 100° C. 0.2mL of a 1 M solution of Te in trioctylphosphine (TOP) was diluted in 10mL oleylamine. This Te solution was quickly injected into the HgCl₂solution; the temperature dropped to 80° C. and the solution turnedblack. The nanocrystal solution was heated at 80° C. for 5 minutes andthen the heat was removed and 15 mL of a quench solution (TCE, 10 vol. %dodecanethiol, ˜1 vol. % TOP) was injected to stop the nanocrystalgrowth. The nanocrystals were isolated by the addition of methanolnon-solvent and centrifugation. The resulting nanocrystal solid wasdissolved in 5 mL TCE.

CdI₃ ⁻ Capping of HgTe Nanocrystals. The nanocrystals were isolated fromTCE by the addition of ethanol non-solvent and centrifugation, and theresulting nanocrystal solid was redissolved in hexane. 2 mL of asolution of HgTe in hexane (˜5 mg/mL) was layered atop 2 mL of 0.1 MCH₃NH₃CdI₃ and stirred for 4 days. The nanocrystals transferred to theNMF, and the non-polar phase was washed with 2 mL hexane 4 times. Thenanocrystal suspension in NMF was washed with a solution of 400 μLtoluene and 200 μL HMPA, isolated by centrifugation, and redispersed in1 mL NMF.

Example 5

This example describes the formation of HgCl₃ ⁻-capped CdTenanocrystals.

CdTe Nanocrystal Synthesis. CdTe nanocrystals were synthesized asdescribed above.

HgCl₃ ⁻ Capping of CdTe Nanocrystals. A 0.1 M HgCl₃ ⁻ ligand solutionwas created by dissolving 0.5 mmol HgCl₂ (136 mg) and 0.5 mmol NH₄Cl(26.7 mg) in 5 mL of dimethyl sulfoxide (DMSO) at room temperature.

The chloromercurate-CdTe ligand exchange was performed in a nitrogenglove box. A solution of oleic-acid capped CdTe nanocrystals in toluenewas washed with ethanol, and the nanocrystals were isolated bycentrifugation. The nanocrystals were redispersed in hexane to create 2mL of a 25 mg/mL solution. This nanocrystal solution was layered atop 2mL of the 0.1 M NH₄HgCl₃ solution and stirred until phase transferoccurred (˜2 hours). The hexane layer was removed, and the polar phasewas washed with hexane 3 times. Then the nanocrystals were isolated byaddition of 1 mL toluene and 0.5 mL HMPA to the polar phase andcentrifugation. The nanocrystal solid was redissolved in 1 mL DMSO.

The solutions were deposited onto glass substrates and annealed to formpolycrystalline mercury cadmium telluride films. PXRD patterns weremeasured for films formed from sintered HgCl₃ ⁻-capped CdTe nanocrystals(FIG. 6A) and from sintered oleic acid-capped CdTe nanocrystals (FIG.6B). The PXRD patterns show that the HgCl₃ ⁻-capped CdTe sintered muchmore significantly at 200° C. than did the oleic acid-capped CdTe evenat 300° C. The mean grain size for the HgCl₃ ⁻-capped CdTe film wasapproximately 28 nm and the grains were zinc blende phase. The meangrain size for the oleic acid-capped CdTe film was approximately 7 nmand the grains were wurtzite phase.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method of forming a semiconductor film, themethod comprising: forming a solution of halometallate-cappedsemiconductor nanocrystals, the halometallate-capped semiconductornanocrystals comprising: semiconductor nanocrystals, each semiconductingnanocrystal having a surface; and halometallate ligands bound to thesurfaces of the semiconductor nanocrystals; forming a coating of thesolution comprising the halometallate-capped semiconductor nanocrystalson a substrate surface; and annealing the coating, wherein thenanocrystals are sintered to form the semiconductor film.
 2. The methodof claim 1, wherein the halometallate ligands act as grain growthpromoters for the semiconductor nanocrystals during the annealing of thecoating.
 3. The method of claim 1, wherein the semiconductornanocrystals are Group II-VI nanocrystals.
 4. The method of claim 3,wherein the Group II-VI nanocrystals comprise a metastable phase of theGroup II-VI semiconductor.
 5. The method of claim 3, wherein the GroupII-VI nanocrystals comprise CdTe nanocrystals, HgSe nanocrystals, HgTenanocrystals, HgS nanocrystals, Hg_(x)Cd_(1-x)Te nanocrystals,Hg_(x)Cd_(1-x)S nanocrystals, Hg_(x)Cd_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)Te nanocrystals, Cd_(x)Zn_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)S nanocrystals, CdSe nanocrystals, CdS nanocrystals, ZnSnanocrystals, ZnSe nanocrystals, ZnTe nanocrystals, or mixtures of twoor more thereof, where 0<x<1.
 6. The method of claim 3, wherein theGroup II-VI nanocrystals are CdTe nanocrystals.
 7. The method of claim1, wherein the halometallate ligands comprise halometallates of metalsselected from group 12 or group 13 of the periodic table of theelements.
 8. The method of claim 1, wherein the halometallate ligandsare selected from CdCl₃ ⁻anions, CdCl₄ ²⁻anions, CdI₃ ⁻anions, CdBr₃⁻anions, CdBr₄ ²⁻anions, InCl₄ ⁻anions, HgCl₃ ⁻anions, ZnCl₃ ⁻anions,ZnCl₄ ²⁻anions, and ZnBr₄ ²⁻anions, and mixtures of two or more thereof.9. The method of claim 6, wherein the halometallate ligands compriseCdCl₃ ⁻anions.
 10. The method of claim 1, wherein the semiconductornanocrystals comprise core-shell nanocrystals; the shells of thecore-shell nanocrystals comprise a Group II-VI semiconductor; and thesemiconductor film comprises the cores of the nanocrystals dispersed ina matrix comprising the sintered shells of the nanocrystals.
 11. Themethod of claim 10, wherein the cores of the core-shell nanocrystalscomprise a Group III-V semiconductor.
 12. The method of claim 10,wherein the Group II-VI semiconductor is CdTe, HgSe, HgTe, HgS,Hg_(x)Cd_(1-x)Te, Hg_(x)Cd_(1-x)S, Hg_(x)Cd_(1-x)Se, Cd_(x)Zn_(1-x)Te,Cd_(x)Zn_(1-x)Se, Cd_(x)Zn_(1-x)S nanocrystals, CdSe, CdS, ZnS, ZnSe,ZnTe, or mixtures of two or more thereof, where 0<x<1.
 13. The method ofclaim 1, wherein coating a solution comprising the halometallate-cappedsemiconductor nanocrystals on a substrate surface comprises doctorblading or spray-coating the solution comprising thehalometallate-capped semiconductor nanocrystals onto the substratesurface.
 14. The method of claim 5, wherein the Group II-VI nanocrystalscomprise CdTe nanocrystals, HgSe nanocrystals, HgTe nanocrystals, HgSnanocrystals, Hg_(x)Cd_(1-x)Te nanocrystals, Hg_(x)Cd_(1-x)Snanocrystals, Hg_(x)Cd_(1-x)Se nanocrystals, Cd_(x)Zn_(1-x)Tenanocrystals, Cd_(x)Zn_(1-x)Se nanocrystals, Cd_(x)Zn_(1-x)Snanocrystals, ZnS nanocrystals, ZnSe nanocrystals, ZnTe nanocrystals, ormixtures of two or more thereof, where 0<x<1.
 15. The method of claim 8,wherein the halometallate ligands are selected from CdCl₄ ²⁻anions, CdI₃⁻anions, CdBr₄ ²⁻anions, ZnCl₃ ⁻anions, and ZnBr₄ ²⁻anions, and mixturesof two or more thereof.
 16. The method of claim 8, wherein thehalometallate ligands are HgCl₃ ⁻anions.
 17. A method of forming anoptoelectronic device, the method comprising: forming a solution ofhalometallate-capped semiconductor nanocrystals, thehalometallate-capped semiconductor nanocrystals comprising:semiconductor nanocrystals, each semiconducting nanocrystal having asurface; and halometallate ligands bound to the surfaces of thesemiconductor nanocrystals; forming a coating of the solution comprisingthe halometallate-capped semiconductor nanocrystals on a surface of afirst electrode; annealing the coating, wherein the semiconductornanocrystals are sintered to form a photoactive, light-absorbingsemiconductor film; forming a layer of charge transporting material overthe photoactive, light-absorbing semiconductor film; and forming asecond electrode on the layer of charge transporting material.
 18. Themethod of claim 17, wherein the semiconductor nanocrystals comprise CdTenanocrystals, HgSe nanocrystals, HgTe nanocrystals, HgS nanocrystals,Hg_(x)Cd_(1-x)Te nanocrystals, Hg_(x)Cd_(1-x)S nanocrystals,Hg_(x)Cd_(1-x)Se nanocrystals, Cd_(x)Zn_(1-x)Te nanocrystals,Hg_(x)Cd_(1-x)Se nanocrystals, Hg_(x)Cd_(1-x)S nanocrystals, CdSenanocrystals, CdS nanocrystals, ZnS nanocrystals, ZnSe nanocrystals,ZnTe nanocrystals, or mixtures of two or more thereof, where 0<x<1. 19.The method of claim 17, wherein the semiconductor nanocrystals are CdTenanocrystals.
 20. The method of claim 17, wherein the halometallateligands are selected from CdCl₄ ²⁻anions, CdI₃ ⁻anions, CdBr₄ ²⁻anions,ZnCl₃ ⁻anions, and ZnBr₄ ²⁻anions, and mixtures of two or more thereof.21. The method of claim 17, wherein the halometallate ligands are HgCl₃⁻anions.
 22. The method of claim 18, wherein the Group II-VInanocrystals comprise CdTe nanocrystals, HgSe nanocrystals, HgTenanocrystals, HgS nanocrystals, Hg_(x)Cd_(1-x)Te nanocrystals,Hg_(x)Cd_(1-x)S nanocrystals, Hg_(x)Cd_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)Te nanocrystals, Cd_(x)Zn_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)S nanocrystals, ZnS nanocrystals, ZnSe nanocrystals, ZnTenanocrystals, or mixtures of two or more thereof, where 0<x<1.
 23. Themethod of claim 19, wherein the halometallate ligands comprise CdCl₃⁻anions.
 24. A method of forming a field effect transistor, the methodcomprising: forming a solution of halometallate-capped semiconductornanocrystals, the halometallate-capped semiconductor nanocrystalscomprising: semiconductor nanocrystals, each semiconducting nanocrystalhaving a surface; and halometallate ligands bound to the surfaces of thesemiconductor nanocrystals; forming a coating of the solution comprisingthe halometallate-capped semiconductor nanocrystals on a surface of agate dielectric layer; annealing the coating, wherein the semiconductornanocrystals are sintered to form a semiconductor film that provides aconducting channel layer for the field effect transistor; forming asource electrode on the semiconductor film; forming a drain electrode onthe semiconductor film; and forming a gate electrode on the gatedielectric layer.
 25. The method of claim 24, wherein the semiconductornanocrystals comprise CdTe nanocrystals, HgSe nanocrystals, HgTenanocrystals, HgS nanocrystals, Hg_(x)Cd_(1-x)Te nanocrystals,Hg_(x)Cd_(1-x)S nanocrystals, Hg_(x)Cd_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)Te nanocrystals, Cd_(x)Zn_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)S nanocrystals, CdSe nanocrystals, CdS nanocrystals, ZnSnanocrystals, ZnSe nanocrystals, ZnTe nanocrystals, or mixtures of twoor more thereof, where 0<x<1.
 26. Halometallate-capped Group II-VInanocrystals comprising: Group II-VI nanocrystals selected from thegroup consisting of CdTe nanocrystals, HgSe nanocrystals, HgTenanocrystals, HgS nanocrystals, Hg_(x)Cd_(1-x)Te nanocrystals,Hg_(x)Cd_(1-x)S nanocrystals, Hg_(x)Cd_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)Te nanocrystals, Cd_(x)Zn_(1-x)Se nanocrystals,Cd_(x)Zn_(1-x)S nanocrystals, ZnS nanocrystals, ZnSe nanocrystals, ZnTenanocrystals, or mixtures of two or more thereof, where 0<x<1, eachnanocrystals having a surface; and halometallate ligands bound to thesurfaces of the Group II-VI nanocrystals, wherein the halometallateligands are anions having one of the formulas MX₃ ⁻, MX₄ ⁻, and MX₄ ² ⁻,where M is an element selected from group 12 of the periodic table and Xis a halide atom.
 27. The nanocrystals of claim 26, wherein the GroupII-VI nanocrystals are CdTe nanocrystals.
 28. A method of making thehalometallate-capped Group II-VI nanocrystals of claim 26, the methodcomprising forming a solution comprising organic ligand-capped GroupII-VI nanocrystals and the anions under conditions that facilitate theexchange of the anions with the organic ligands capping the Group II-VInanocrystals, whereby the halometallate-capped Group II-VI nanocrystalsof claim 19 are formed.